In the context in which rotary regenerative air preheaters are used it is frequently desirable to obtain air streams, which have been heated by regenerative heat exchange from exhaust gases, at different temperatures or under different pressure conditions. This occurs for example where it is desired to heat separately primary and secondary air for coal fired furnace installations. The quantity and the pressure of the primary and the secondary air have to be regulatable independently of each other, the temperature of the primary air, for example, being varied in dependence on the moisture content of the fuel being used.
Conventionally the heating of these separate air streams has been done in two separate exchangers. A high capital cost is involved and obviously space is required for the accommodation of two preheaters, which nowadays can be of extremely large size ranging up as large as 15 meters in diameter.
Although this is the conventional solution, various attempts have been made before now to provide for the separate degree of heating of different components passing through a rotary regenerative heat exchanger.
There have been proposals for example for different media which are to be heated up during the heat exchange to be led separately to or from the regenerative mass.
For example in U.S. Pat. No. 1,858,508 (corresponding to German Pat. No. 484548) there is seen a stationary mass above and below which are stationary supply and outlet channels for three different media. One is a heating medium and the other two are media to be heated. The separation of the three is achieved by having at each end face of the mass rotating radially extending walls which effectively divide the mass at any one time into three sectoral flow compartments and by having above and below those walls respectively, plates rotating with the walls and in which are cut openings at respectively different radii so that these openings are in constant communication with the respective stationary supply or output ducts, the mouths of which are annuli of different radii.
Now this arrangement, at least as far as we are aware has never been put into practice, most probably for two main reasons. The first is that, because of the inherent needs of the exchanger with regard to the volumes of the mass being exposed to the different media, the rotating parts will have to be rotationally unsymmetrical and this will impose both a static and a dynamic load on the bearings. The dynamic load will be aggravated by any pressure differences existing between the different media (which under modern operating conditions, at least, there frequently are).
The second and perhaps more serious difference is that concerned with assuring seals between the various parts. There is an unsymmetrical temperature distribution within the regenerative mass which results in uneven deformations and stresses which will cause almost insoluble problems in regard to the sealing between the end faces of the mass and the rotating walls. This will be particularly serious (as will also be the problem of sealing between the plates and stationary annular ducts) in the very large size regenerative preheaters (up to 15 meters in diameter) which are used today. Our conclusion is that the type of approach represented by German Pat. No. 484548 with stationary ducts and stationary mass with gas flows being separated and directed by rotational radial walls and apertured plates is not and never has been a practical proposition.
A further and different approach to the problem has been seen in German Pat. No. 1242788 where to provide an air output of different temperatures, the regenerative mass was divided into two axially separated parts, with a rotating outlet duct having a sector shaped hood sweeping over the upper face of the upper part of the divided mass and a much smaller, coaxially arranged, pipe having a small sectoral shaped opening in communication with the output from the upper face of the lower part of the mass. This sectoral opening of the central pipe is not in direct contact with the lower part of the mass but merely permits part of the regeneratively heated air emitted from that part to be abstracted through the central pipe.
Therefore although there will be air outputs of different temperatures, there can be no precision of control as to their relative temperatures of their relative pressure or flow rates.
More recently, a proposal due to the assignees of the present application and seen in DTOS (German Laid open specification) No. P24 18 902 is one in which primary and secondary air streams are supplied concentrically with each other to separate annular volumes in the stationary regenerative mass by means of rotating and symmetrical double cowls. At least one of these cowls has a radially outer annular sectoral portion divided by an arcuate wall from a radially inner sectoral portion, thus to separate gas flows into two different radial components of the regenerative mass. This form of construction had proved successful in practice but there is a disadvantage in that when there is a difference between the amounts of primary and secondary air being passed to the heater, the flow gases (which are exchanging heat with radially separated portions of the mass by virtue of the division of the cowl or cowls) will leave the regenerative mass at very different temperatures. This results in uneven temperature and velocity distributions of those gases after they have left the mass, which may unfavourably effect the operation of dust removal installation which are connected in series to the preheater. Furthermore it has been found that this form of construction may need very precise design for the boiler in order that the subsequent flow of flue gas and air may efficiently work the regenerator. This cannot be achieved in all installations and so a solution has been sought which would liberate the designer from such constraints. This is seen in the present invention.
Also in the prior art is DTOS (German Laid open Specification) No. P2523841, in the name of the assignees of the present application. Here a rotatable hood has at one edge an extra, radially extending and sectoral-shaped, chamber. The hood and the chamber go to coaxial separate ducts at the throat of the hood. This extra chamber is a sluice chamber which has the function of withdrawing impure gas from sectors of the mass which in succession lie below it, so that that gas will not be brought into the air flow to contaminate it.
There is no concept here, and no structural possibility, of directing separate air flows through distinct heating paths.
Lastly is a proposal seen in U.S. Pat. No. 3,799,242 where a rotating mass regenerator is equipped with three different media-conveying stationary ducts at each end face. As that specification says, this is a version of the tri-sector arrangement known from the then prior art -- compare German Pat. No. 484548, to which U.S. Pat. No. 1,858,508 corresponds and to which U.S. Pat. No. 3,799,242 is very close in its arrangement of the stationary ducting. There is however no equivalence in the present context between stationary ducting at the end face of the mass and the rotatable cowls seen in the preheater of the present invention, since, for example, reorganization of the duct work to permit rotation, the provision of an enclosing stationary duct within which the rotating hoods may work are involved. Furthermore one would get the type of unsymmetricality, distortion and sealing problems described in connection with German Pat. No. 484548 and for the same reasons, if one merely attempted to rotate the end-covers of the mass of U.S. Pat. No. 3,799,242, while keeping that mass stationary.